The present invention relates to an improved ink jet printing apparatus which eliminates a cause of distorted images.
In such an apparatus, a sheet of paper is moved relative to an ink ejection head. A typical arrangement is to wrap the sheet around a drum and rotate the drum for scanning. Each time a scan line is printed, the ejection head is moved parallel to the axis of the drum by one increment to print the next scan line. An alternating drive signal is applied to the ejection head to cause a jet of ink to be ejected therefrom which separates into droplets at a certain position. An alternating charging signal is applied to a charging electrode disposed at said position to charge the droplets where it is desired to print a dot on the sheet. The charged droplets are deflected by deflecting electrodes onto the sheet. Where it is not desired to print a dot, the charging signal is not applied and the droplets are not deflected and hit a gutter.
It is known to intially vary the phase of the drive signal relative to the charging signal until it is detected that the droplets are charged. However, due to fluctuations in the rotational speed of the drum and the like it has occurred in the prior art that adjacent scan lines are displaced from each other in the scan direction, producing a distorted image. Although it is theoretically possible to reduce this problem to a negligible level by increasing the frequency of the drive and charging pulses, it is not possible in actual practice due to the limited operating speed of the apparatus.